1. Field of the Invention
The present invention relates to a device to eliminate the ice formed on the surface of a wall.
The present invention shall be described chiefly with respect to an application to the de-icing of an optical or radio-electrical window, for example the window of a radome. However, it will easily be understood that an application such as this is in no way restrictive and that a great many applications to the de-icing of other types of walls, especially in aeronautics, may be considered without going beyond the scope of the invention.
2. Description of the Prior Art
Essentially, when a deposit of ice formed on a surface has to be removed, the most common method used (apart from methods using compounds based on hydrophobic agents) consists in blowing hot air on to the wall to be de-iced, or in heating this wall by the Joule effect through metal wires deposited on the surface of this wall or embedded within it.
The major drawback of a system such as this lies in the fact that the heating wire actually heats only the part closest to it for, in almost every case, the material of the wall has low thermal conductivity as is typically the case with windows made of glass, whether mineral or organic.
This drawback is accentuated by the unfavorable ratio generally existing between the diameter of the wire and the spacing between two consecutive wires, whereas this ratio should be as small as possible when the wall is a window, in order to minimize the disturbances that might be introduced by the heating system.
Another drawback lies in the large amount of heat lost through conduction and radiation on the front and rear faces of the wall that is to be de-iced. This factor makes it necessary to have additional electrical power to compensate for the losses.
Furthermore, the heating is not instantaneous and, if it is to be properly efficient, the time that it requires to reach thermal equilibrium must become greater as and when the temperature is further away from the melting temperature of ice.
Furthermore, should the wall be a radio-electric window such as a radome window, a number of constraints come into play, notably the fact that, so as not to disturb the wave transmitted by the radome-shielded radar, the metal wires should be, firstly, deposited perpendicularly to the polarization of the wave transmitted (which limits the choice of the polarization). Secondly, the wires should be spaced out with respect to one another by a distance determined by a relationship known to those skilled in the art (Proceedings of the IRE, Vol 49, 1961, pp. 427-447). This distance would be of the order of 8 mm approximately for a radar working at 4 GHz through a window.
This latter constraint thus necessitates the matching of the de-icing system with the working frequency of the radar covered by the radome, thus severely restricting the possibilities of choice of the characteristics of this radome.
Finally, to ensure maximum reliability, especially in the case of a radar, it would be necessary to provide for a standby energy source for the de-icing system. Given the substantial power needed for de-icing (it is estimated that, for a 4 m.sup.2 window corresponding to the surface of a radar radome on the ground transmitting at 4 GHz, it is necessary to provide about 2 to 3 kW per m.sup.2, that is 8 to 12 kW for the window), this results in installations in which cost and overall dimensions become prohibitive.
One of the aims of the invention is to propose a device that overcomes all of these drawbacks by means of a system using no longer the Joule effect but the mechanical vibrations produced by acoustic transmitters, notably acoustic transmitters based on ferroelectric polymers, deposited at the location of the window of the radome.
The major promising feature of this technique lies in the fact that the effect is mechanical and, therefore, does not necessitate any heating time or any permanent operation (intermittent operation will suffice): this is a considerable advantage in terms of energy consumption.
Besides, it will be seen that the invention enables the making of walls, the entire surface of which is active (and no longer walls that are active only locally, as in the case of heating wires), the consequence of which will be to considerably increase the efficiency of conversion of electrical power into acoustic power and, hence, the ultimate efficiency of the system.
It will also be seen that the invention can be used to make a radio-electric window that is transparent to microwaves, irrespectively of the transmission frequency of the radar, hence a window which, by virtue of this quality, does not require any special matching for a given frequency, unlike prior art de-icing systems using heating wires.
Finally, it will be seen that the invention can be adapted to plane walls as well as to walls of different shapes, for example corrugated walls or alternately corrugated and plane walls, according to a structure that enables the further amplification of the mechanical deformation produced by the acoustic wave, and hence the efficiency of the system.